NITRIFICATION AND OCCURRENCE OF NITROBACTER BY MPN-PCR IN LOW AND HIGH NITRIFYING CONIFEROUS FOREST SOILS

The nitrification of three coniferous forest soils was investigated: a podzol (Fontainebleau, France) with low N deposition and no nitrate a ccumulation, an acid mull (Haldon, UK) with low N deposition and moder ate nitrate accumulation, and a podzol (Wekerom, The Netherlands) with high N deposition and high nitrate accumulation. Twenty-one months in situ lysimeter experiments and short-term and long-term in vitro incu bations were performed to respectively establish the status of NO3- ac cumulation and potential nitrification of these soils. These complemen tary approaches allowed to conclude that the absence of NO3- accumulat ion in Fontainebleau soil was effectively due to a lack of nitrifing a ctivity in the whole profile while the high NO3- accumulation observed in the Wekerom soil essentially resulted from an active nitrifying ac tivity in the Oh horizon of this soil. For the Haldon soil, the inadeq uacy between the lysimeter and the short-term in vitro experiment was discussed. Surprisingly, relatively high densities of the nitrite-oxid iser Nitrobacter (enumerated by PCR-MPN technique) were found in all t he studied horizons of the three soils. Moreover, a long-term incubati on experiment showed that inhibition of nitrification in the Oh Fontai nebleau soil could be removed after submitting the soil samples to con stant conditions during 82 days. This demonstrates that in these soils , ammonium-oxidisers (as well as nitrite-oxidisers) are present and th at extrinsic factors were involved in the inhibition of nitrification.